It is necessary to dress, or to true abrasive grinding tools of the aforesaid kind from time to time, in order to restore the tool to its original geometry, which shall be rotationally symmetrical with a given generatrice, and to remove all abrasive grains and to expose fresh grains which present sharp cutting edges.
To this end a grinding machine is normally equipped with a dressing tool which incorporates an extremely hard material, often diamond, and which may have a varying form or geometry. In many instances, the dressing tool is in the form of a rotatable diamond cylinder or disc. When the abrasive grinding tool is to be dressed, or trued, the tool is moved from its grinding location to a dressing location, where it is dressed. The tool is then moved back to its grinding location, and grinding is continued.
A rotatable dressing tool of this kind is normally mounted on a separate journal bearing, which of necessity must be robust and reliable in order not to lose accuracy when dressing the abrasive grinding tool. As before-mentioned, the dressing tool is normally in the form of a cylinder, wheel or disc having an outer or peripheral diamond lining, or a coating of some other highly wear-resistant material.
The time taken to move the grinding tool from its grinding location to the aforesaid dressing location is of relatively long duration and prolongs the time taken to complete a grinding cycle, besides constituting an interruption in the grinding cycle. In addition hereto, the dressing-tool bearing, upon which the aforesaid high demands are placed, constitutes a cost-increasing complication which markedly increases the total costs of the grinding machine.
The conventional method of mounting a dressing tool also gives rise to a further problem which is not readily resolved, namely that a temperature difference often prevails between the location at which the grinding tool engages the workpiece and the location at which it engages the dressing tool. The machine components supporting the grinding tool, the workpiece and the dressing tool are subjected to changes in linear measurement as a result of these temperature differentials, which may have a comparatively serious effect on grinding accuracy. It will be noted that grinding work of the kind envisaged here often demands a measuring accuracy of a fraction of a micron.
In U.S. Pat. No. 4,259,940 (Asano et al) there is described a grinding machine tail-stock fitted with a dressing tool, said grinding machine comprising
(a) a holding device, such as a chuck for rotatably supporting a workpiece;
(b) a tool slide for rotatably supporting a grinding tool;
(c) means for effecting axial relative displacement of the tool slide and/or a chuck in a manner to move the grinding tool into and out of a workpiece engagement location, and to feed the grinding tool in an axial direction;
(d) means for effecting transverse relative movement of the tool slide and/or the chuck for engagement of the grinding tool with the workpiece, and for transverse feed relative thereto;
(e) a dressing tool mounted in connection with the holding device for co-rotation therewith.
The grinding wheel of this grinding machine has a rather big diameter. A truing wheel and a dressing roll are coaxially fixed on a work support shaft of a work head in juxtaposed relation with each other. A wheel feed mechanism is used to infeed a grinding wheel against the truing wheel so as to establish between the dressing wheel and the grinding wheel a gap of predetermined distance toward which free abrasive grains acting as a dressing agent are supplied. This arrangement has some of the above-mentioned drawbacks.
U.S. Pat. No. 4,112,624 (Uhtenwoldt et al) describes a shoe-type grinding machine having a work head including an annular platen which rotates and engages one end of the workpiece and includes shoes which engage the outer surface of the workpiece. A dressing wheel in the form of an annulus is mounted on the workpiece clamp. This arrangement belongs to a different type of grinding machines. Due to the fact that the axis of the annular platen is neither coaxial with the axis of the workpiece nor with that of the grinding tool the result of the grinding operation will be negatively affected. Therefore, such a machine cannot be used when there are high requirements on the grinding operation.